diff --git a/m4/guestfs-ocaml.m4 b/m4/guestfs-ocaml.m4
index 46e6c47d8..9823c40b0 100644
--- a/m4/guestfs-ocaml.m4
+++ b/m4/guestfs-ocaml.m4
@@ -199,7 +199,7 @@ EOF
 ])
 
 dnl Flags we want to pass to every OCaml compiler call.
-OCAML_WARN_ERROR="-warn-error +C+D+E+F+L+M+P+S+U+V+Y+Z+X+52-3-6"
+OCAML_WARN_ERROR="-warn-error +C+D+E+F+L+M+P+S+U+V+Y+Z+X+52-3-6 -w -6"
 AC_SUBST([OCAML_WARN_ERROR])
 OCAML_FLAGS="-g -annot $safe_string_option"
 AC_SUBST([OCAML_FLAGS])